Update records in Zoho forms - bug or error?

Hi,

I'm trying to build a simple app that just adds points to students based on the type of networking events they attend. The following are the forms/reports used:

1. Students - list of student records with id, name and e-mail (and points accumulated)
2. Events - list of events with event_id, name and points
3. Event attendance - Form where I will select an event and enter student ID so that student gets the required points

In the event attendance form, once I submit the form, I want the students record (in the Students list) to get updated with the additional points earned. For this, I use the Form Actions -> On Success workflow to script the update. However, I get an error in the actual form.. My code is like the following:

stu2 = Students[ID == input.student_id] ; (collection variable for student record)
su2.Points = input.points_earned ( also try it by just assigning some value, say 9 -> stu2.Points = 9; )

The code shows no script errors, but when I run the app and submit the forms, I get the error log which reports:

Error in executing the Update record tasks.... Unable to update stu2.Points ... Unable to update the value 9.

I've made sure that

users can update fields, but the error is really strange!!!

Any help would be appreciated.... :)